Abstract
Abstract: :
Purpose: The pattern-pulse multifocal visual evoked potential (PPMVEP) (James, 2003 IOVS) is a method for the rapid concurrent testing of multiple visual field locations. It uses a temporally sparse stimulation consisting of pulses of pattern contrast, rather than the more commonly used rapid contrast reversal, and appears to maintain the visual system in a high contrast-gain state, producing greater response strength and signal-to-noise ratio (SNR). We here study the dependence of signal strength and SNR on presentation rate, pulse duration, and alternation or not of the polarity of pattern contrast . Methods: Stimuli were presented laid out as a 56 region cortically-scaled dartboard of total diameter 48o, monocularly, at 75Hz frame rate. A 4x4 checkerboard pattern was pulsed in contrast independently in each region at a mean rate which varied from 1 to 20Hz, for pulse durations varying from 13 to 52ms, with contrast polarity constant or alternating between pulses. Evoked potentials were recorded from 12 subjects, at 3cm above inion vs 4cm below, for 4 repeats of 55s runs of each stimulus condition. Elementary response waveforms were quantified by RMS from 50-120ms, and noise RMS was taken from 400-800ms, beyond the response window. The multiplicative effects of stimulus parameters were estimated by multiple regression of decibel (dB) signal, noise and SNR on the stimulus factors. Results: Signal strength increases as mean rate of presentation decreases, by around 1.25dB per Hz, reaching a plateau at 2Hz. Noise however also increases as mean rate decreases, and the SNR tradeoff produces an optimal plateau at 2 to 4Hz. Signal increases as pulse duration increases from 13 to 26ms, then levels. Noise does not vary significantly, and SNR is optimal at 26ms. Alternation of polarity produced a small increase in signal and SNR. Conclusions: The contrast gain control mechanism appears to be directly driven by the mean rate of presentation, with gain increasing until a plateau is reached at around 2Hz. Hence this is a relatively rapid adaptation phenomonon, with near complete recovery of gain within around 0.5 seconds. Taking gain as response divided by pulse duration, contrast gain declines also as pulse duration increases. Optimal SNR is obtained at the temporally sparse rates in the range 2 to 4Hz, and represents a 4 times improvement in power SNR, compared to faster presentation rates, meaning one quarter the recording time produces a given quality of data. This saving in recording time significantly increases the clinical viability of multifocal evoked potentials.
Keywords: electrophysiology: clinical • perimetry • visual fields